Rh antigens reside in Rh polypeptides of the red cell membrane and form one of the most complex polymorphisms in humans. They are clinically important, accounting for the most common incompatibility found in blood transfusion and hemolytic disease of the newborn. They appear to be essential membrane components because red cells devoid of all Rh antigens (Rh/null phenotype) manifest stomatocytosis, shortened in vivo survival, and multiple membrane abnormalities. Our objectives are to unravel the structure/function relationships of the Rh system and delineate the genetic mechanisms for its phenotypic diversity. The specific aims are: 1) To clone and map the entire RH locus and establish the physical order and transcriptional direction of its gene members. 2) To determine the molecular basis of different Rh antigens and phenotypes with emphasis on how DNA recombination and other genetic mechanisms modulate gene structure and cause antigenic variation. 3) To study the expression of a novel Rh transcript that may characterize the d (D-negative) haplotypes and determine the mechanism that results in its activation. 4) To study differential splicing of the Rh genes and determine its role in the onset and regulation of Rh gene expression during erythroid development. 5) To identify the Rh/null and Rhmod genes and determine whether the primary defect is in the Rh genes themselves or in the Rh-related candidate genes. The proposed studies promise to provide insights into structure/function relationships of Rh proteins and the genetic mechanisms leading to one of the most polymorphic human systems. The knowledge gained will deepen our understanding of clinical problems associated with blood transfusion and hemolytic reactions and provide a basis for the clinical service to use molecular approaches in transfusion medicine.